The amount of imported natural gases is growing with the demand on natural gases in China, and the dependence on foreign countries is correspondingly increasing, which seriously threatens the energy security in China. The shale gas resources are very rich in China, and the efficient development of the shale gas is of great significance for alleviating the contradiction between the energy supply and demand, adjusting the energy structure and ensuring the energy security. The amount of the shale gas resources in China is expected to reach 1.0×1014 m3, which is twice of the amount of conventional natural gases. According to its recoverable resources and exploitation potential, the shale gas is expected to become a third important unconventional oil and gas resource following the coal bed gas and the tight sandstone gas.
Due to the characteristics of low porosity and low permeability of the shale gas reservoirs, reservoir fracturing and permeability increasing have to be carried out for the development of the shale gas. Currently, the United States mainly adopts the hydraulic fracturing technology to exploit the shale gas, but this technology consumes a lot of water resources and seriously pollutes the underground water; meanwhile, the content of clay in the shale in China is generally high, and the water-expandable characteristics of the shale will also affect the effect of reservoir transformation. In addition, China's proven shale gas reserves are mostly distributed in basins and mountainous areas where water resources are scarce, and the development of the shale gas in those areas also faces challenges in water resources.
When the temperature and pressure of carbon dioxide are above 31.10° C. and above 7.38 MPa, respectively, carbon dioxide will reach a supercritical state. Since the supercritical carbon dioxide has gas-like diffusivity as well as liquid density and solvency, while combining the characteristics such as low viscosity and low surface tension, it has excellent flow, penetration and transfer properties and can replace fresh water as a fracturing fluid. There are very few theoretical and experimental studies on the supercritical carbon dioxide fractured shale, especially corresponding test methods are absent for studying a process of artificial fracture initiation and extension during supercritical carbon dioxide fracturing, as well as information such as fracture length, fracture width and fracture extension speed of an artificial fracture during fracturing.
Therefore, by virtue of years of experiences and practices in related industries, the inventor proposes a visualized supercritical carbon dioxide fracturing physical simulation test method to overcome the defects of the prior art.